1. Field of the Invention
The present invention relates to a vapor compression system and more particularly to the use of a plurality of fixed and variable type expansion devices in the vapor compression system.
2. Description of the Related Art
A vapor compression system typically includes a compressor, a first heat exchanger, an expansion device, and a second heat exchanger fluidly connected in series. Other components such as accumulators or economizing heat exchangers are also well-known and may be employed with the vapor compression system but are not essential for the operation of the vapor compression system. In operation, the compressor typically compresses a refrigerant vapor from a low suction pressure to a higher discharge pressure. The refrigerant is cooled in the first heat exchanger. In a subcritical vapor compression system, the refrigerant is converted from a gas state to a liquid state in the first heat exchanger which may be referred to as a condenser. The high pressure liquid refrigerant exiting the condenser passes through the expansion device where the pressure of the liquid is reduced. The low pressure liquid refrigerant is then converted to a vapor in the second heat exchanger, commonly referred to as an evaporator. The conversion of the refrigerant to a vapor requires thermal energy and the evaporator may be used to cool a secondary heat medium, e.g., air that may then be used to cool a refrigerated cabinet or the interior space of a building. The low pressure refrigerant vapor is then returned to the compressor and the cycle is repeated. Other applications, such as heat pump and water heater applications may utilize a vapor compression system for the heat generated by the first heat exchanger.
Other known types of vapor compression systems include transcritical vapor compression systems. In such transcritical systems, the refrigerant is compressed to a supercritical pressure by the compressor and is returned to the compressor at a subcritical pressure. When the refrigerant is at a supercritical pressure, the liquid and vapor phases of the refrigerant are indistinguishable and the first heat exchanger is commonly referred to as a gas cooler. After cooling the refrigerant in the gas cooler, the pressure of the refrigerant is reduced to a subcritical pressure by the expansion device and the low pressure liquid is communicated to the evaporator where the refrigerant is converted back to a vapor.
When carbon dioxide is used as a refrigerant, the vapor compression system must typically be operated as a transcritical system. The use of carbon dioxide as a refrigerant also generally requires the use of a discharge pressure that is considerably higher than the discharge pressure used with conventional refrigerants that can be used in a subcritical system. This relatively high pressure required when using carbon dioxide as refrigerant may result in greater stress and wear on the individual components which form the vapor compression system. For example, when a variable expansion valve is employed as the expansion device in a transcritical vapor compression system employing carbon dioxide as a refrigerant, the valve seat of the expansion valve may be subject to a relatively high rate of wear and negatively impact the length of its useful life.